This invention relates to a method of and apparatus for modifying a video signal and, more particular, to such a method and apparatus wherein a predetermined signal is inserted into the horizontal blanking interval of the video signal such that a video picture can be reproduced directly from the modified video signal but a satisfactory video picture cannot be reproduced if the modified video signal is recorded and then played back on a conventional type of video signal recorder.
In many instances, it is desirable to modify a video signal so as to prevent or discourage unauthorized use thereof. For example, with the recent introduction of video signal recording systems, such as video tape recorders (VTR), it may be desirable to modify the video signals in a manner which would prevent the reproduction of a satisfactory video picture in the event that such video signals are recorded and then played back, but the modification should be of the type which would permit a satisfactory video picture to be reproduced directly if the modified video signals are broadcasted. As another example, it would be advantageous if master video recording media, such as video tapes, could be recorded in a manner such that a satisfactory video picture can be reproduced directly from such media but not from a copied version thereof. Hence, the recorded information, such as a television program, which may be expensive to produce, could not be copied without authorization, thereby minimizing the risk of "piracy."
Video signal modification for the purpose of avoiding piracy can be attained by using unique encoding and decoding techniques, but such techniques are quite expensive, and suitable decoding apparatus must be provided with each television receiver in order to display a satisfactory video picture from the received, encoded video signal.
One proposal for modifying a video signal so as to avoid the piracy thereof is described in British Pat. No. 1,028,025, published May 4, 1966. In that system, the black levels in a video signal are detected and used to generate "decoy" signals having the nature of horizontal synchronizing signals, which decoy signals are coincident with the detected black levels. Unless these decoy signals are removed from the video signal, they are erroneously interpreted by a conventional television receiver as being horizontal synchronizing signals. Hence, the horizontal oscillator included in the television receiver cannot be synchronized properly with the correct horizontal synchronizing signals included in the modified video signal. Since this proposal depends entirely on the presence of black levels in many horizontal line intervals, there is the possibility that, in the absence of such black levels in a particular televised scene, this video signal modifying technique may not be successful.
Another proposal for modifying a video signal to avoid piracy is described in copending application Ser. No. 778,709 filed Mar. 17, 1977, which has been filed by the inventors of the present invention and which has a common assignee. In that other proposal, the vertical synchronizing pulses included in the vertical blanking interval are modified in a fashion such that the usual vertical oscillator included in a television receiver nevertheless can be synchronized with the modified vertical synchronizing pulses, but the modified vertical synchronizing pulses cannot be detected in a conventional video signal recorder, such as a VTR. Since the detection of such vertical synchronizing pulses is necessary for the proper operation of the VTR, the inability to detect such pulses results in the recording and reproduction of a video picture having poor quality.
The present invention is directed to an alternative technique for modifying a video signal so as to avoid piracy.